Method of making low-void filament wound structures



NOV. 25, 1969 J, PAUL, JR

METHOD OF'MAKING Low voID FILAMENT WOUND STRUCTURES Filed Sept;

2 Sheets-Sheet 1 JAMES T. PAUL, JR. INVENTOR.

g w wmw AGENT J. 'T. PAUL, JR

Nov. 25, 1969 METHOD OFMAKING LOW-VOID FILAMENT WOUND STRUCTURES FiledSept. 12, 1966 2 Sheets-Sheet 2 JAMES T. PAUL, JR. INVENTOR.

United States Patent 0 3,480,499 METHOD OF MAKING LOW-VOID FILAMENTWOUND STRUCTURES James T. Paul, Jr., Wilmington, DeL, assignor toHercules Incorporated, Wilmington, Del., a corporation of Delaware FiledSept. 12, 1966, Ser. No. 578,605 Int. Cl. B65h 54/04; B32b 1/02 US. Cl.156-175 3 Claims ABSTRACT OF THE DISCLOSURE A process for makinglow-void filament wound structures wherein the roving is directed ontothe rotating winding mandrel of a winding machine by means of a rovingguide that traverses a path along the mandrel and directs the rovingonto the mandrel in accordance with the predetermined pattern, theroving being passed through a vacuum chamber directly into immersion ina resin supply whereby the roving is substantially void-free prior towinding.

The present invention relates to a method of and apparatus for forming afilament wound article, and particularly to forming such an article witha reduced number and volume of voids.

The filament wound articles to which the present invention relates havea wall consisting of a wound fiber and a binding material, the fiberbeing of glass or other material that is characterized by a high tensilestrength and light weight or, stated difierently, a very highstrength-toweight ratio, and the binding material being a curablethermosetting resin, such as an epoxy polymer, which impregnates thewindings and, when cured, becomes permanently set and thus binds thewound fibers into a selfsupporting structure. These articles aredesigned primarily for use in applications requiring light weight aswell as the capacity for withstanding high pressures or other stresses,such applications being for example a rocket case or the boom of acherry-picker, or the capacity for withstanding high electrical stressesas required for use in circuit breaker tubes.

In making filament wound articles, one method that has been widely usedcommercially is the so-called wetwinding process wherein the fiber inthe form of a roving is first passed through a supply of the resinousbinding material, which may be any suitable thermosetting resin that isliquid at room temperature and can be cured at temperatures within thetolerance of the fiber. The fiber or roving is thus wetted with thebinding material and is thereafter wound, usually under tension, on amandrel in accordance with the predetermined pattern. The bindingmaterial is then cured, after which the wound structure is separatedfrom the mandrel, leaving a selfsupporting structure having wallsconsisting of the wound fiber or roving bound together by the hardenedbinding material.

Filament wound articles heretofore produced by the wet-winding processmay generally be characterized as including voids in the windings ofbetween four and ten percent by volume, the voids being a local absenceof binding material between adjacent filaments, either the adjacentfilaments in a single turn of the roving or the adjacent filaments ofsuccessive turns of the roving. These voids have significant adverseefiects upon the physical properties of the structure and particularlyupon the iuter filamentary shear strength, that is, shear strengthbetween adjacent filaments or longitudinally thereof, which strength canbe increased by twenty-five to one hundred percent by elimination of thevoids. In addition to in- 3,480,499 Patented Nov. 25, 1969 creasing theinter-filamentary shear strength, the reduction of voids in a filamentwound structure also increases the dielectric strength thereof, therebyadapting the same for use in applications such as in the circuit breakertubes of a power distribution system where electrical properties as wellas high strength, light weight, inertness and/or thermal properties areimportant. Another significant characteristic of a low-void filamentwound structure is its optical clarity. When the number of voids arereduced to a minimum, and assuming a compatible selection of glass fiberand binding material, the transparency of the structure is greatlyimproved, thereby rendering the structure useful in applications such aspressure tanks where seeing into the tank is important.

A primary cause of the voids is air carried into the wound structure bythe roving and which cannot escape. This air is adhered to the surfaceof the fibers, trapped between the filaments thereof, and dissolved ordispersed in the resinous binding material with which the roving iswetted. The rope pump action of the roving as it moves rapidly into thesupply of the binding material tends to increase to saturation theamount of air dissolved in the resin and also causes foaming orfrothing, which forms fine bubbles that adhere to the surface of theroving, The air not only prevents wetting of the filaments by thebinding material during impregnation of the roving out also expandsduring curing of the binding material because of the heat, whichexpansion tends to force the binding material out of the structure andto enlarge the void which is then fixed in the enlarged condition by thecuring or setting of the binding material. The escape of the air fromthe inter-filamentary spaces is inhibited by the fineness of thefilaments and by the encapsulation of the air bubbles by the bindingmaterial during impregnation. The viscosity of the resinous bindingmaterial also has a direct relation to the entrapment of air bubbles bythe material, which viscosity can be reduced by heat but preferablyshould be reduced as little as possible in this manner since the heatalso reduces the pot-life of the resin.

A further source of voids in wound structures are errors in the windingpattern which leave gaps between adjacent fibers of successive turns ofthe roving, or at crossing points of the windings in the windingpattern, or at the discontinuities that occur about inserts, such as thewafers for nozzle or port connections or reinforcing materials. Voids atthe inserts can also result from poor wetting of the insert by thebinding material whereby there is insufficient binding material providedfor filling the resulting gaps.

Voids also result from an inadequate supply of resin in the woundstructure either because of improper coating or impregnation of theroving or because of handling the impregnated roving before winding orof the wound structures after winding. Scraping or otherwise removingexcess material from the roving before winding may remove too much ofthe material so that there is an insufficient amount to fill theinter-filamentary spaces in the wound structure. In scraping the woundstructure while it is still wet and soft, the localized pressure of thescraping tool may squeeze out too much of the resin so that when thepressure is suddenly removed, the windings quickly recover to theirnormal condition and produce voids that fill with air. A further sourceof voids is the drainage of the binding material from the wet woundstructure. The extent of the drainage depends upon the length of timethat the wound structure was allowed to stand, its orientation, and theviscosity of the resinous binding material, which of course istemporarily reduced by heating during the initial stages of the curing.

The amount of voids in a filament wound structure also varies with otherfactors that affect the penetration of the binding material into theroving, such as the compactness of the roving or the condition of thefiber. As examples, the fiber may have a finish that inhibits orpromotes wetting by the binding material or may have filaments that havebecome bonded together during storage and that form barriers to thepenetration of the resin into the underlying fibers.

Prior commercial efforts to eliminate voids in a filament Woundstructure have centered around the use of tension on the roving duringwinding to compact the windings and thus force out the air, the use ofheat to reduce the amount of air that is dissolved in the resin and tofacilitate the escape of the air pumped into the resin supply by therope-pump effect, and the use of pressure during curing to reduce thebubbles by physically compressing them and by increasing the solubilityof the air in the resin to dissolve the air from the bubbles. There hasalso been an experimental proposal to reduce voids by mounting thewinding apparatus in a vacuum chamber. None of these approaches has beencompletely successful and all have disadvantages, such as for example,heating reduces the pot life of the resin, or pressure curing requiresmassive and expensive equipment and is only partially successful inremoving the voids, or winding in a vacuum requires expensive equipmentand is difficult to control because of the inaccessibility, bothvisually and physically, of the windings.

The primary object of this invention is to provide a process andapparatus for making a filament wound structure having a reduced numberof voids and therefore having increased strength, particularlyinter-filamentary shear strength and compressive strength, and havingimproved optical clarity and electrical properties, which process andapparatus involve a minimum investment in equipment, is simple tooperate, is economical, de-

pendable and trouble-free in operation, and which is effective inproducing the desired results.

In accordance with this invention, the above objects have been attainedby treating the roving prior to winding to remove or to reduce the airadhered to the surface or encapsulated or dissolved in the resin, asWell as that which is trapped within the inter-filamentary spaces of theroving. Specifically, the invention resides in passing the rovingcontinuously from a vacuum chamber directly into immersion into a resinsupply for impregnating the roving with resin. With existing windingmachines substantally void-free filament wound structures operating inthe normal manner, can be produced simply by substituting the presentresin supply for the usual resin cups.

One of the significant features of this invention is that, in additionto the removal of the air from the surface and the inter-filamentaryspaces of the roving, the roving passes from the vacuum directly intothe supply of impregnating resin so that there is no air carried intothe resin supply by the rope-pump action of the roving. The resin supplythus remains clear and relatively airfree during the entire operation,which is in contrast to the condition of the resin supply in a normalwet-winding operation wherein the resin supply becomes completelyclouded by frothing or the dispersion of minute air bubbles throughoutthe supply almost immediately at the beginning of the winding operation.

With the above and other objects in view, a preferred embodiment of thepresent invention is hereinafter described with referenoe to theaccompanying drawings, in which:

FIG. 1 is a fragmentary elevational view of a winding machine embodyingthe present invention.

FIG. 2 is a fragmentary sectional view taken substantially on the line22 of FIG. 1.

FIG. 3 is a detailed vertical sectional view longitudinally of thevacuum impregnator for the roving of the machine of FIG. 1.

FIG. 4 is a fragmentary detailed view in section of a modification ofthe present invention.

With reference to the drawings, there is illustrated somewhatschematically in FIG. 1 a winding machine having end supports 1 and 2with winding spindles 3 and 4 for releasably receiving the rotatingmandrel 5 upon which the filament wound structure is to be wound. Torotate the mandrel 5, it is coupled for rotation to the spindle 3 whichin turn is driven by drive means (not shown) housed in the end support1.

A roving carriage 6 is adapted to traverse back and forth along themandrel 5 to feed resin-impregnated roving onto the mandrel. Theillustrated carriage 6 comprises a platform 7 having bearing lugs 8 thatare slidably mounted on a pair of supporting rods 9 extending betweenthe end supports 1 and 2 and having the axes thereof parallel to theaxis of the mandrel 5, whereby the platform 7 is supported for movementin a path parallel to the axis of the mandrel 5. For driving thecarriage 6 back and forth along the rods 9, there is an arm 10 dependingfrom the platform 7 and having a vertical slot 11 that receives thedrive pin 12 of a drive chain 13. The chain 13 is entrained about a pairof sprockets 14 that are journaled on a cross piece 15 extending betweenthe end supports 1 and 2, the sprocket 14 adjacent to the end support 1being driven by a "chain 16 entrained about a sprocket 17 (FIG. 2) andabout a drive sprocket (not shown) housed within the end support 1. Thechain 13 with its pin 12 cooperates with the arm 10 in the manner of ascotch yoke to drive the carriage 6 back and forth as the chain isdriven.

The platform 7 carries a plurality of spools 18 of roving which aremounted on frictioned spindles 19 upstanding from the platform 7 andwhich impose a selected resistance to rotation of the spools 18 wherebya predetermined tension is imposed on the roving as it is drawn from thespools. From the spools 18 the roving R passes to a roving guide 20 of atake-up mechanism 21 that is designed to take up the slack in the rovingas the direction of travel of the carriage 6 is reversed at the end ofeach stroke, that is, as the pin 12 passes around one of the sprockets14. The take-up mechanism 21 comprises an arm 22 having the guide 20 atone end thereof and being pivotally mounted at its other end to abracket 23 upstanding from the platform 7. A constant bias is imposedupon the arm 22 to move the free end thereof, i.e., the end carrying theguide 20, in the direction to take up slack in the roving, which istoward the dotted line position in FIG. 2. The means for biasing the arm22 comprises an air cylinder 24 that is pivotally mounted at one end tothe bracket 23 and has the piston rod 25 thereof pivotally connected tothe arm 22. When the roving is under normal tension as it is when beingdrawn from the spools 18 during winding, the arm 22 is deflected to thefull line position illustrated in FIG. 2.

From the roving guide 20 at the end of the take-up arm 22, the roving Rpasses into an impregnator 26 comprising an entry chamber 27, a vacuumchamber 28, and an exit chamber 29. The entry chamber 27 has anexternally threaded inlet tube 30 that extends through a threadedopening 31 in the top of the chamber 27 and a cylindricalextension 32extending laterally from one side thereof. The extension 32 includes anaxial bore 33 that serves as a roving passage and which is provided witha resin seal 34 that provides for passage of the roving from the entrychamber into the vacuum chamber 28, but which restricts the leakage ofresin into the vacuum chamber and which seal 34 constitutes the inletseal for the vacuum chamber. As illustrated, the seal 34 may comprise ashort section of soft rubber tube 35 inserted into the end of the bore33 and adapted to be collapsed or pinched onto the roving by a pair ofopposed slides 36 seated in slots in the cylindrical extension 32 andmovable toward and away from each other by means of adjustment screws37. The inlet tube 30 has toroidal guides 38 that are semicircular incross-section and are mounted at both the input and the output endsthereof to minimize the abrasion of the roving as it passes the edges ofthe tube upon entering and leaving the same. The inlet tube is threadedinto the entry chamber until the lower end thereof is substantially atthe level of the midpoint of the bore 33 whereby the roving will passdirectly into the bore 33 without contacting the edges thereof.

The exit chamber 29 is substantially the same as the entry chamber 27and includes an outlet tube 39 with toroidal roving guides 40 at the topand bottom thereof, and a lateral extension 41 having a bore 42 throughwhich the roving passes and which is provided with a seal 43 forpreventing resin from leaking from the exit chamber into the vacuumchamber 28 while at the same time permitting the roving to pass from thevacuum chamber 28 into the exit chamber 29.

The vacuum chamber 28 comprises a tubular section that surrounds and issecured to the extensions 32 and 41 of the entry chamber 27 and exitchamber 29, the section, for example, being threaded onto the sectionsas shown in FIG. 3. A vacuum line 44 is connected to the vacuum chamber28 at one end and at its other end is connected to a source of vacuum(not shown). The amount of vacuum imposed on the vacuum chamber 28 isnot critical and it has been found that a vacuum in which the absolutepressure is in the range of ten to twenty millimeters of mercury isadequate as well as economical to maintain.

The entry chamber 27 and its input tube 30 are filled with resin to thelevel indicated at 45 and are maintained full to a level within theinlet tube 30 whereby the roving R will be wetted within the resinbefore contacting the roving guide 38 at the bottom of the inlet tube30. In the same manner as in the usual wet-winding processes, as theroving moves into the inlet tube 30, it pumps entrained air into theresin and also sets up a circulation pattern in the resin so that theresin is thoroughly mixed with air bubbles. Most of this air can beremoved in the same manner as heretofore used in wet-winding processes,that is, by heating the resin to reduce its viscosity and thusfacilitate escape of the trapped air. With the present system, theresidue of this air, or all of it when using resins that cannot tolerateprolonged heat, is readily removed in the vacuum chamber 28. While thereis some resin impregnation of the roving in the entry chamber 27, theprimary purpose of the chamber 27 is to wet the roving with resin sothat it is well lubricated before it passes over the lower guide 38 andthrough the seal 34, thereby reducing abrasion and assisting inmaintaining the integrity of the seal 34.

Upon passing through the seal 34 and into the vacuum chamber 28, theroving R is subjected to the vacuum and is immediately purged of the airadhered to the surface thereof and trapped in the inter-filamentaryspaces thereof. At the same time, because of the reduced pressure, airdissolved in the resin or encapsulated therein is also released.

From the vacuum chamber, the roving R passes through the seal 43 intothe exit chamber 29 and exits from the chamber 29 at the roving guide 40at the top of the outlet tube 39. The chamber 29 is normally filled withresin to a level above that of the bore 42, that is, to a level withinthe outlet tube 39, such as the level 46 as illustrated. Inasmuch as theroving is substantially evacuated of air in the vacuum chamber 28, it isimmediately thoroughly impregnated by the resin in the exit chamber 29which is at atmospheric pressure. The resin in the exit chamber may, ifdesired, be de-gassed prior to use. Since the roving entering thechamber is substantially evacuated and, entering from a vacuum, there isno tendency for the roving to entrain or pump air into the resin. Also,since the roving is partially wetted with resin from the entry chamber27, which resin was substantially flashed in the vacuum chamber, thereis a reduced amount of resin from the exit chamber required to saturatethe roving.

In order to help maintain the level of the resin in the entry chamber 27and in the exit chamber 29, resin reservoirs (not shown) may beconnected to them. A further advantage of such a reservoir is that itprovides a relatively wide and easy to fill opening, whereas theopenings defined by the guides 38 and 40 at the upper ends of the inlettube 30 and outlet tube 39 are designed primarily for lclontrolling thelead of the roving and may be quite sma In operation, in the usualmanner, the roving carriage 6 is moved back and forth along the mandrel5, which, as it rotates, draws roving from the supply spools 18, throughthe impregnator 26 and out the roving guide 40 at the top of the outlettube 39. The pattern in which the roving is laid on the mandrel is afunction of the speed of rotation of the mandrel relative to the speedat which the carriage 6 is traveling. Preferably, the speed .of themandrel 5 is variable so that the Winding may proceed at a relativelyrapid rate, while the speed is substantially reduced when the directionof travel of the roving carriage 6 is reversed at the end of eachstroke, to prevent excessive twisting of the roving.

Substantially void-free roving has been produced in accordance with thisinvention at roving speeds of up to two hundred feet per minute. Suchroving, when wound into a filament wound structure, produces asubstantially void-free structure.

In addition to the fact that substantially void-free windings areproduced by the present invention, a further advantage is that itrequires a minimum investment in equipment. Most winding machines inoperation today have a roving carriage that traverses a path along themandrel and carry a supply of roving that is led to the mandrel througha resin cup. The present invention can be practiced on such equipment byreplacing the resin cup with a vacuum impregnator for the roving asherein disclosed. It will, of course, be apparent that the specificdetails of the mechanism disclosed are not essential to the invention inits broadest aspects and, in particular, the exit chamber 29 could bemade to deliver the roving downwardly rather than upwardly bysubstituting a resin chamber such as that which forms the subject matterof the Young Patent No. 3,025,205 for that disclosed herein, thesubstitution requiring only that the roving pass directly from thevacuum chamber into the resin chamber at a point below the level of theresin and therefore passes directly into the resin.

While the outlet tube 39 may be open to the atmosphere at its upper endas illustrated in FIG. 3, so that the roving passes out of the resinsupply, this arrangement with some resins permits the roving to carry anexcess of resin over that which can be used in the winding. This excessnot only represents a waste of resin but also presents a problem incollecting it and in preventing it from being thrown about. To reducethe quantity of resin on the roving t a minimum in excess of that whichis required to saturate the windings, the outlet tube may be providedwith a flap or flexible seal 47 as illustrated in FIG. 4. The seal 47overlies the open area defined by the guide 40 at the outlet of the tube39 and is held in position by a shoulder screw 48 that is threaded intoa spider 49 in the tube 39. The roving R is designed to pass outwardlybetween the guide 40 and the seal 47 whereby the relatively lightpressure of the seal on the roving tends to wipe the roving and toreturn some of the resin thereon to the reservoir. A further advantageof the seal 47 is that it closes the top of the tube 39 and thus permitsfilling the tube 39 completely with resin. In this manner, the tube 39can be connected to an external resin reservoir, as by the conduit 50,which has a head greater than that in the tube 39 without overfilling orspitting the resin from the tube.

I claim:

1. In a method of making a filament wound structure from a roving thatis characterized by high tensile strength and light weight and a bindingmaterial that consists of a curable thermosetting resin, said methodcomprising the steps of impregnating a continuous length of said rovingwith said resin by passing the same through an impregnator, guiding theresin-wet roving to a roving guide, moving the roving guide andimpregnator along a path parallel to the axis of the rotating mandrel ofa winding machine, actuating said roving guide along said path inaccordance with a predetermined pattern for winding the roving onto saidmandrel in accordance with said predetermined pattern, and curing saidresin to bind the wound roving into a filament wound structure, andwherein, in the step of impregnating said roving, said roving is passedcontinuously in the impregnator through a seal into a vacuum chamber forsubstantially evacuating the air from the inter-filamentary spaces andfrom the surface thereof, from the vacuum chamber through a sealdirectly into immersion in a resin supply, and from said resin supply tothe atmosphere, whereby the resinwet roving guided to said roving guideis substantially void-free.

2. The method of making a filament wound structure in accordance withclaim 1 wherein said roving is preimpregnated with resin prior topassing from said vacuum chamber through said seal and into immersion insaid resin supply.

3. The method of making a filament wound structure in accordance withclaim 1 wherein said roving is passed from the atmosphere into immersionin an initial resin supply prior to being passed through the seal intothe vacuum chamber.

References Cited UNITED STATES PATENTS 3,025,205 3/1962 Young 156-1692,125,364 8/1938 Waldron 118-5O 2,848,354 8/1958 Daley 117 119 XR PHILIPDIER, Primary Examiner US. Cl, X.R.

